The present invention relates to a cross-flow filtration method and a cross-flow filtration device.
In recent years, microfiltration and ultrafiltration technologies are increasingly applied to various fields such as protein adsorbents, water purification, air cleaning, structures for deodorization equipment, denitrification equipment and exhaust gas equipment, ozone removal, removal of various viruses, materials for clean rooms, gas separation, alkaline elution, asbestos monitoring, detection of parasites, IR spectroscopy of particles in the air, analysis of algae in a river or sea, erythrocyte deformability tests, chemotaxis and culture chambers, scanning electron microscopes, radioactive assays, exfoliative cytology and X-ray fluorescence spectrometry. In the various applications, fine-structure filters are applied to such purposes as separation, purification, recovery and concentration.
However, in the case of separating fine particles with a fine-structure filter, microfiltration and ultrafiltration suffered from such problems as generation of resistance in the flow of a fluid passing therethrough due to a deposit layer (cake layer) formed under the influence of concentration polarization and an increased resistance due to clogging in a membrane filter, and these problems hampered the commercialization of the filter of this type.
A well-known filtration system for solving these problems is a so-called dead-end filtration system in which the whole fluid to be filtered is passed through a filtering medium and a cake layer to separate fine particles contained in the fluid.
However, in order for the fluid to pass through the filtering medium and the cake layer to separate material suspended in the fluid, the dead-end filtration system requires a pressure overcoming the resistance of the permeating fluid caused by the cake layer.
Therefore, when applied to microfiltration or ultrafiltration, the dead-end filtration system suffered from a decreased amount of filtration (amount of permeation) per membrane area and per unit time.
The applicant of the invention has provided techniques for improving the amount of filtration per membrane area and per unit time compared to the conventional dead-end filtration system by adopting a cross-flow filtration system which involves passing a feed to be filtered parallel to the membrane surface of a membrane filter and passing fluid through the membrane filter to the opposite side while suppressing formation of a cake layer by a support of the membrane filter provided on the side on which the feed flows (see JP 04-145929 A, JP 04-190834 A and JP 04-190835 A, for example).
However, since the membrane surface is parallel to the flow of the feed, the cross-flow filtration system is low in filtration accuracy and has difficulty in considerably improving the amount of filtration per membrane area and per unit time, and improvement methods are under study.